The mechanisms involved in murine virus-induced leukemogenesis are being investigated. Studies on the pathogenesis of different strains of the virus and the immunologic responses to them are in progress. The two funtions of virus synthesis and erythroid differentiation in the tissue-cultured erythroleukemic cells appear to be under separate mechanisms of control and the regulation for each is being studied. The amount as well as the biochemical and biophysical characteristics of the virus synthesized by variant clones after treatment with different types of inducers of maturation are being compared, as is the degree of differentiation and malignancy in syngeneic hosts.